This invention relates to a thermal head driver circuit for a thermal printer.
Recently, various office automation apparatuses are used to improve business efficiency. Among various office automation apparatuses, thermal printers, typically thermal transfer printers, are becoming increasingly popular because they are quieter, have a relatively simple structure and are easy to maintain.
Relatively recent thermal printers are capable of producing clear colors and a recent trend has been to employ these printers as color printers.
Considerable research and development has been invested in intermediate tone expression by thermal printers. As typical approaches, there are an area graduation process, in which intermediate tones are expressed by binalizing image data by a dither method or the like, and a density graduation process, in which the heat generation of each heat generation element of a thermal head is controlled according to image data for each picture element, thereby varying the area of a print dot to vary the image density.
In the mean time, with a thermal printer it is liable that when the printing speed is increased, accurate print image density control can not be obtained due to heat storage in heat generation elements of the thermal head. This is so because with an increase of the printing speed the heat energization cycle is shortened so that a new energization cycle is started while the heat generated in the previous cycle has not yet be sufficiently radiated. If such heat storage proceeds, considerable heat different is provided between a continually energized heat generation element and a newly energized heat generation element, resulting in a wide density difference between print dots.
In order to eliminate such adverse effects of the heat storage, it is considered to control the amount of energy supplied to each heat generation element of the thermal head. More specifically, it is thought to supply an energization pulse having a short duration compared to the normal energization pulse to a heat generation element, which has been repeatedly energized for a number of times and also to a newly energized heat generation element adjacent to heat generation elements which have been repeatedly energized. With this method, it is possible to preclude the adverse effects of the heat storage. However, it is necessary to vary the pulse duration according to the status of printing. Further, energization pulses corresponding in number to graduations which are greater in number than the actually printed graduations. Therefore, the circuit construction becomes complex.
In the case of the density graduation process the duration of the pulse supplied to the heat generation element has to be finely controlled to provide a large number of graduations. Particularly, to obtain a high speed density graduation printing the pulse duration should be controlled more accurately in view of the influence of the heat storage.
A prior art thermal head driver circuit for effecting such an energization pulse duration will be described with reference to FIG. 1. Image data 2 representing the density of each picture element is supplied to series input terminal S.sub.IN of shift register 1. The image data is 4-bit data for each picture element, and image data for each bit are fed continuously. For example, the first bit (i.e., least significant bit) of image data for each picture element is successively fed to shift register 1, and the second to fourth (i.e., most significant) bits of each picture element image data are successively fed to shift register 1. Image data of one bit fed to series input terminal S.sub.IN of shift register 1 is transferred through stages thereof in synchronism to a clock signal 3. Heating section 9 of the thermal head consists of a plurality of heat generation resistors. When image data 2 for picture elements corresponding in number to the number of the heat generation resistors have been fed to and transferred through shift register 1, the supply of the clock signal 3 is discontinued to discontinue the transfer of image data 2. At this time, a latch signal 4 is fed to clock terminal CK of each flip-flop of latch circuit 5, whereby image data output from series output terminal S.sub.OUT of each stage of shift register 1 is latched in a flip-flop of each stage of latch circuit 5. Output terminal Q of the flip-flop of each stage of latch circuit 5 is connected to a first input terminal of AND gates constituting gate circuit 7.
When print signal 6 is generated and fed to a second input terminal of AND gates of gate circuit 7, each AND gate is enabled when and only when the output data of a corresponding flip-flop of latch circuit 5 is "1". Therefore, when image data is "1", a transistor constituting driver circuit 8 is turned on according to a print signal 6, causing current to flow to a heating resistor of thermal head heating section 9. Thermal printing is thus done by the heat which is generated. The above operation takes place repeatedly for the first (least significant) to the fourth (mast significant) bits of image data of each picture element.
When the pulse durations of print signals T.sub.1 to T.sub.4 for the first (least significant) to fourth (most significant) bits are set such that T.sub.1 : T.sub.2 : T.sub.3 : T.sub.4 =1:2:4:8, as shown in FIG. 2A, the density of each picture element can be expressed in 16 different graduations with 4-bit image data. For example, when the first (lest significant) to fourth (most significant) bits of image data are 1011, respectively, energization pulses as shown in FIG. 2B are supplied to the heat generation resistors.
FIGS. 3A to 3E are timing charts showing the operation of a line thermal head with heat generation resistors arranged linearly. In this case, if all the heat generation resistors of the line thermal head are energized at a time, an extremely large energization current is caused. This means that power source which has a sufficiently large current capacity is needed. Accordingly, the heat generation section of the thermal head is divided into two blocks so that the section is energized block by block. FIG. 3A shows image data 2. Image data 2 for the first bit (n being the number of heat generation resistors of one block) is transferred through shift register 1 according to a clock signal 3 shown in FIG. 3B. When the transfer of image data of the first bit is ended, a latch signal 4 as shown in FIG. 3C is generated, whereby the image data of the first bit is latched in latch circuit 5. When a print signal 6 (T.sub.1) is generated as shown in FIG. 3D, current is caused to flow through only the heat generation resistor which is connected to latch circuit flip-flops, in which data " 1" among the first bit image data is latched. In this way, the printing is effected.
Further, the transfer of image data for the second bit (including the input to shift register 1) is effected while image data for the first bit is being printed. When the printing with the first block is ended, the printing with the second block is effected. That is, the transfer of image data of the first bit in the second block is effected while the image data for the fourth bit in the first block is being printed.
With the above prior art thermal head driver circuit, the energization pulse can be pulse duration modulated into 16 different pulses. In the case of the two-block driving, however, it is necessary to transfer data eight times from an external device to the shift register. For this reason, a considerable time is necessary for the printing of data for one line. This drawback will now be discussed in greater details.
A case is now considered, in which a line thermal head (4,096 dots) for an A4 size width with a line printing cycle period of 1 msec. and a resolution of 16 dots/mm is used. In this case, the printing period for one block is 0.5 msec. During this period of 0.5 msec., 2,048-bit data has to be transferred four times from the external device to the shift register. This means that the frequency of the clock signal for the transfer of data through the shift register is as high as 16 MHz. Actually, it is impossible to cause transfer of data through the shift register at such a high speed. Accordingly, it is thought to increase the blocks to eight blocks, divide all the image input terminals of the thermal head into eight blocks, and supply image data as parallel data to each block. In this case, the clock frequency can be reduced to 4 MHz. At present, however, the highest available thermal head clock frequency is 2 MHz which is far lower than 4 MHz. When such a thermal head is used, it is necessary to provide 16 image input terminals. Therefore, an increased number of lines are necessary for supplying data to the thermal head. Or, a buffer memory is necessary for dividing one line data into 16 divisions and holding the divided data to supply the data to the thermal head. This increases the scale of the electric circuitry for supplying data to the thermal head. For this reason, it has been impossible to realize a high speed thermal printer with a one line printing time of 1 msec.
Further, even if it is possible to transfer data at a clock frequency of 4 MHz, one cycle of data transfer takes 128 .mu.sec. when the thermal head is driven by 8-block driving. The time necessary for one cycle of printing should not be made shorter than this data transfer period. This is so because if an energization pulse shorter than one data transfer cycle period, there will occur a printing-free period between adjacent printing cycles. In such a case, it is very troublesome to compensate for the heat storage. If the pulse duration is set to be equal to one data transfer cycle period (i.e., 128 .mu.sec.), eight printing cycles can be effected in 1,024 msec. In this case, therefore, a line printing cycle period of 1 msec. can be realized only in case when the pulses T.sub.1 to T.sub.4 have the same pulse duration. If the pulses T.sub.1 to T.sub.4 are all of the same pulse duration, only four different pulse durations can be obtained, and it is impossible to effect which the graduation printing on fine energization pulse duration control for precluding the influence of the heat storage.
As has been shown, with the prior art thermal head driver circuit it has been impossible to simultaneously realize high speed and multiple graduation printing free from the influence of the heat storage.